Aobo Guo and Claire C. Ashmead

Hathaway Brown School, Shaker Heights, Ohio

Kim K. de Groh

Glenn Research Center, Cleveland, Ohio

Edward A. Sechkar

ASRC Aerospace Corporation, Cleveland, Ohio

Effect of Solar Exposure on the Atomic Oxygen 

Erosion of Hubble Space Telescope AluminizedTefl on Thermal Shields

NASA/TM—2012-217646

September 2012
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Aobo Guo and Claire C. Ashmead

Hathaway Brown School, Shaker Heights, Ohio

Kim K. de Groh

Glenn Research Center, Cleveland, Ohio

Edward A. Sechkar

ASRC Aerospace Corporation, Cleveland, Ohio

Effect of Solar Exposure on the Atomic Oxygen 

Erosion of Hubble Space Telescope AluminizedTefl on Thermal Shields

NASA/TM—2012-217646

September 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

10th International Space Conference on Protection of Materials and Structures From the Space

Environment (ICPMSE-10J)

cosponsored by ITL, MDA, CSA, JAXA, Kobe University Graduate School of Engineering,

and the Society for Promotion of Space Science

Bankoku-Shinryokan, Okinawa, Japan, June 12–17, 2011
Acknowledgments

The authors would like to thank Marc Van Eesbeek of ESA for supplying the HST BSTS Weld section. Also, they appreciate the 

technical advice provided by Bruce Banks of Alphaport, Inc. at NASA Glenn Research Center. Finally, the authors would like to 

thank Patty Hunt, Director of Research at Hathaway Brown School, for making it possible for the student co-authors to collaborate 

with NASA on this and other research projects.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217646 1 

Effect of Solar Exposure on the Atomic Oxygen Erosion of Hubble 

Space Telescope Aluminized-Teflon Thermal Shields 

Aobo Guo and Claire C. Ashmead 

Hathaway Brown School 

Shaker Heights, Ohio 44122 

Kim K. de Groh 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Edward A. Sechkar 

ASRC Aerospace Corporation 

Cleveland, Ohio 44135 

Abstract 

When exposed to low Earth orbital (LEO) environment, external spacecraft materials degrade due to 

radiation, thermal cycling, micrometeoroid and debris impacts, and atomic oxygen (AO) interaction. 

Collisions between AO and spacecraft can result in oxidation of external spacecraft surface materials, 

which can lead to erosion and severe structural and/or optical property deterioration. It is therefore 

essential to understand the AO erosion yield (Ey), the volume loss per incident oxygen atom (cm3

/atom), 

of polymers to assure durability of spacecraft materials. The objective of this study was to determine 

whether solar radiation exposure can increase the rate of AO erosion of polymers in LEO. The material 

studied was a section of aluminized-Teflon (DuPont) fluorinated ethylene propylene (Al-FEP) thermal 

shield exposed to space on the Hubble Space Telescope (HST) for 8.25 years. Retrieved samples were 

sectioned from the circular thermal shield and exposed to ground laboratory thermal energy AO. The 

results indicate that the average Ey of the solar facing HST Al-FEP was 1.910–24 cm3

/atom, while the 

average Ey of the anti-solar HST Al-FEP was 1.510–24 cm3

/atom. The Ey of the pristine samples was 

1.6–1.710–24 cm3

/atom. These results indicate that solar exposure affects the post-flight erosion rate of 

FEP in a plasma asher. Therefore, it likely affects the erosion rate while in LEO. 

Introduction 

When exposed to low Earth orbit (LEO), external spacecraft materials can degrade as a result of 

radiation, thermal cycling, micrometeoroid and debris impacts, and atomic oxygen (AO) interaction. AO 

is the most predominant chemical species in LEO, formed when ultraviolet (UV) radiation at wavelengths 

below 0.243 m causes diatomic oxygen to photodissociate (Ref. 1). The average impact energy of AO at 

International Space Station (ISS) altitudes (400 km above the Earth) and at spacecraft orbital speeds of 

7.7 km/s is approximately 4.5 eV (Ref. 2). Collisions of spacecraft with the residual AO can result in 

oxidation of the external spacecraft surfaces. Oxidation of susceptible materials, such as polymers, can 

result in fragmentation of the polymer chains and formation of volatile material causing erosion and 

resulting in severe structural and/or optical property deterioration. Atomic oxygen erosion of polymeric 

components, such as thermal control films and solar array blankets, can be a serious threat to spacecraft 

performance and durability. It is, therefore, essential to understand the AO erosion yield (Ey), the volume 

loss per incident oxygen atom (cm3

/atom), of polymers to have knowledge of materials durability relevant 

to spacecraft applications (Ref. 2). Aside from AO, UV radiation can also cause significant alteration to 

polymeric surfaces, posing additional problems for spacecraft. Absorption of a photon of UV radiation by 
NASA/TM—2012-217646 2 

an organic molecule raises the molecule to an excited state, potentially stimulating bond dissociation and 

formation of free radicals that can cause further scission and crosslinking (Ref. 3). Because the solar 

facing sides of the thermal shields is hotter than the anti-solar sides, there can be increased mobility of the 

free radicals allowing them to react more readily thus causing more polymer chain damage that probably 

manifests itself as a reduction in elongation to failure. If sufficient ultraviolet radiation fluence is 

accumulated the degradation could end up causing the chopped polymer segments to be small enough to 

evaporate thus impacting optical and emittance. It can also cause degradation of mechanical properties, 

discoloration, and altered electrical properties (Ref. 4). Other forms of solar exposure, such as charged 

particle radiation, x-rays from solar flares, and heating and thermal cycling, are additional environmental 

threats that can degrade spacecraft materials. The issue of whether solar radiation can increase the rate of 

AO erosion, hence the AO erosion yield, is of debate in the space community. 

This research addresses the AO erosion of a commonly used spacecraft thermal control material, 

aluminized-Teflon fluorinated ethylene propylene (Al-FEP), specifically whether solar radiation exposure 

increases the rate of AO erosion. This study is unique in that it utilizes Al-FEP that has been exposed to 

the space environment for 8.25 years on the Hubble Space Telescope, material that was then retrieved by 

astronauts during a servicing mission and brought back to Earth. 

Shortly after the Hubble Space Telescope (HST) was deployed into LEO on April 25, 1990, the metal 

bi-stem booms supporting the solar arrays (SA-I) were discovered to be rapidly contracting and 

expanding as the telescope orbited in and out of the Earth’s shadow, causing a thermal-induced jitter 

(Ref. 5). In response to this problem, the European Space Agency (ESA) built new HST solar arrays 

(SA-II) with bi-stem thermal shields (BSTS) comprised of 2 mil (0.051 mm) Al-FEP rings fused together 

into circular bellows shape (with the FEP layer facing space) for thermal insulation (Ref. 5). Astronauts 

replaced the original arrays with the BSTS containing arrays during servicing mission 1 (SM1) in 

December 1991. Figure 1 shows the HST with SA-II and a close-up image of a section of BSTS. During 

the fourth servicing mission (SM3B) in March 2002 after experiencing 8.25 years of space exposure the 

SA-II were replaced with a third set of arrays (SA-III) and the SA-II was brought back to Earth. ESA 

provided a section of retrieved BSTS to NASA Glenn Research Center so that environmental durability 

studies of the Al-FEP could be conducted. 

Because the thermal shields were wrapped around the solar array bi-stems and hence had solar, antisolar, and solar-grazing surfaces, the objective of this experiment was to characterize the AO Ey of 

retrieved HST BSTS Al-FEP with respect to solar angle to see if the Ey varied with solar exposure. The 

solar-facing surfaces were found to be extremely embrittled with through-thickness cracks, while the antisolar-facing side was very ductile, like pristine material (Ref. 5). It was therefore theorized that solarfacing Al-FEP would have a greater erosion yield, due to additional damage to the material induced from 

the solar radiation and/or heating caused by solar exposure. For this study 24 samples were sectioned 

from a single weld of the retrieved HST BSTS Al-FEP, and Ey values versus solar angle were determined 

using a RF plasma asher. 

Figure 1.—Hubble Space Telescope with SA-II photographed in March 

2002 during SM3B with a close-up photograph of a section of BSTS 

(Ref. 5).
NASA/TM—2012-217646 3 

Materials and Experimental Procedures 

Materials 

The BSTS consisted of welded-together FEP rings whose backsides were coated with approximately 

1000-Å thick vapor-deposited aluminum and whose inner and outer diameters (when laid flat) were 6 and 

10 cm, respectively. Using an X-acto knife, the ESA-supplied 20 weld section was separated into 20 

welds (Welds 1-20) by cutting through the inner fused region. Each weld consisted of two pieces of 

Al-FEP seamed at the top; these were then separated and consistently labeled either side A or side B. 

Both sides of Weld 15 were used for conducting this study (Side B in Test 1 and Side A in Test 2, as 

indicated in the Experimental Procedures section below). 

For the purposes of this experiment, the direct solar-facing surface is defined as being 0° and the antisolar-facing surface as 180°, with solar-grazing surfaces as 90° and 270°. Researchers who examined the 

retrieved solar array and sectioned the BSTS 20-weld section stated that the most damaged (cracked) 

region of the BSTS was solar-facing on-orbit (Ref. 6). However, the exact position and thus the specific 

angle of the BSTS sample that was directly solar-facing was not known. Therefore, the 0° position was 

chosen to be the center of the most obviously damaged region. Pristine Al-FEP samples (2 mil thick) 

obtained from Sheldahl and pristine BSTS samples fabricated at the same time as the HST BSTS for preflight environmental durability testing (Ref. 7) were also tested to provide control Ey values. Kapton H 

was used as an exposure reference because of its well-characterized in-space erosion yield 

(3.010–24 cm3

/atom) (Ref. 8). 

Space Environmental Exposure 

The estimated environmental exposure conditions for the SA-II BSTS Al-FEP, installed during SM1 

and retrieved during SM3B, are given in Table 1 (Ref. 5). Details on the computation of number of thermal 

cycles, equivalent sun hours (ESH), albedo solar exposure, x-ray fluence, electron and proton fluence, AO 

ram fluence, and thermal modeling are described by de Groh et al. in Reference 5. The ESH, X-ray fluence 

and AO fluence were adjusted for weld geometry with surfaces 23.5° from sun incidence (Ref. 5). 

AO ram fluences were modeled based on the Mass Spectrometer Incoherent Scatter Model 86 

(MSIS-86) (Ref. 9). The AO fluences for solar-facing and anti-solar facing surfaces were determined by 

multiplying the ram fluence by 0.2528 and 0.3167, respectively, (Ref. 10) and then were further adjusted 

by a factor of 0.399 (cosine 66.5°) from the ram fluence value due to the weld geometry (Ref. 5). 

TABLE 1.—ENVIRONMENTAL EXPOSURE CONDITIONS FOR HST BSTS SURFACES 

Exposure SM1 to SM3B (SA-II) 

Shuttle Missions STS-61 and STS-109 

Mission dates December 1993 and March 2002 

Exposure duration (Yrs) 8.25 

Thermal cycles (#) 45,100 

Temperature range (°C) Anti-solar: –140 and Solar: +43 

ESH, Solar-facing: Direct/Albedo/Total ~19,600/~500/~20,000 

ESH, Anti-solar (albedo only) ~6,200 

X-ray fluence: 1-8 Å/ 0.5-4 Å (J/m2) 63.8/4.0 

Electron fluence (#/cm2),  40 keV 2.08×1013

Proton fluence (#/cm2),  40 keV 2.55×1010

AO fluence (atoms/cm2) 

Ram 1.2×1021

Solar 1.2×1020

Anti-solar 1.5×1020
NASA/TM—2012-217646 4 

Experimental Procedures 

Samples 

For Test 1, sixteen samples were sectioned from HST BSTS Weld 15 Side B at approximately every 

20 to 30°. Eight samples (mostly solar grazing and anti-solar) were sectioned using a 1.1 cm diameter 

circular punch. Eight solar-facing samples were cut by hand using an X-acto knife to avoid cracks. 

Figure 2(a) shows the location where each of the samples was sectioned from Weld 15 Side B. Samples 

were then exposed to thermal energy AO in a Structure Probe, Inc. Plasma Prep II asher for Ey

determination versus on-orbit solar angle. Multiple exposure tests were conducted to evaluate effects of 

Ey versus AO fluence. Seven of the irregularly shaped hand-cut samples were fragile and so were placed 

in small aluminum holders with circular apertures to provide support during handling and exposure, as 

well as to provide smaller exposure windows to AO than the sample holder (described below) provides, 

shown in Figure 2(b). The individual aluminum holders created to mount the irregularly shaped samples 

were exposed to AO prior to use to remove any organic coating that could contaminate the samples and 

contribute to erroneous results pertaining to mass loss. 

For Test 2, eight samples were sectioned from the solar side of Weld 15 Side A. Five of the samples 

were obtained using a 1.1 cm diameter circular punch and three of the samples were cut by hand using an 

X-acto knife. Once again, the three irregularly shaped samples were placed in pre-AO exposed individual 

aluminum holders. Samples in the individual Al holders in both tests were left in the holders during the 

entire test duration and were weighed while in the holders. Three 1.1 cm-diameter pristine BSTS samples, 

three 1.1 cm diameter pristine Al-FEP samples, and two 2.54 cm diameter Kapton H samples were also 

sectioned using circular punches. Samples obtained between 270° clockwise to 90° are considered “solarfacing” samples while those obtained between 90° clockwise to 270° are termed “anti-solar,” 

corresponding with the Sample Angle Template shown in Figure 2(a). Samples at 90° and 270° are 

referred to as “solar-grazing”. It should be noted that the angles of samples sectioned from Side A match 

those of the Side B samples, since samples at a particular angle (such as 60°) were facing the same 

direction on the telescope. 

Mass Measurements 

The erosion yield values obtained for this experiment were determined based on mass loss. Due to the 

hygroscopic nature of Kapton H polyimide, all Kapton H samples were either dehydrated for at least 48 hr 

prior to measuring their masses (as in the case of the Kapton flux test) or prioritized to be immediately 

weighed once taken out of the asher (as in the case of the Kapton witness sample in each test exposure). 

A test was conducted to compare the dehydrated weight of Al-FEP to non-dehydrated weight. Al-FEP 

sample weights were found to not vary significantly when dehydrated, therefore the Al-FEP samples were 

not dehydrated prior to sample weighing. All sample weights were obtained using a Mettler Balance with 

a sensitivity of ±1 μg. 

 (a) (b) 

Figure 2.—(a) Weld 15 Side B after sectioning of 16 test 

samples at specified angles, and (b) an irregularly 

shaped sample from Weld 15 Side A in its Al holder. 
NASA/TM—2012-217646 5 

Ground-Laboratory RF Plasma Asher Exposure 

The samples were exposed to AO in a Structured Probe, Inc. Plasma Prep II asher, which generates a 

plasma by exciting the ambient air with 100 W of continues RF power at 13.56 MHz. The operation 

pressure was between 0.5–1.0×102

 mtorr. The samples were exposed to AO while being held in a metal 

sample holder with a large central opening for a Kapton fluence witness sample (2.54 cm diameter) and 

with 22 smaller sample openings (1.1 in. cm diameter). Figure 3 shows the sample holder loaded with 

Kapton samples for a flux test (described below) along with sample position descriptions noted (A-Y). 

Each test included six individual AO exposures, so that the effect of fluence on Ey could be 

determined. The same sample was always placed in the same position for each incremental exposure. 

Aside from protecting the edges and backs of the samples from the AO plasma and keeping the samples 

from curling during exposure, the holder also provided a well-defined exposure area necessary for further 

calculations. The holder was always placed in the same position in the asher in an effort to ensure the 

same flux (f, atoms/(cm2

s)) for each sample position. 

The 2.54 cm diameter Kapton H fluence witness sample was placed in position Y in the metal holder 

for every test in order to determine the AO effective fluence (F, atoms/cm2). The fluence of each sample 

in every exposure needed to be calculated to compute erosion yield, and finding individual sample 

fluences is discussed in the Flux Tests section below. As stated previously, Kapton H is used as a fluence 

reference because of its well-characterized in-space erosion yield in LEO. The equation to compute the 

exposure fluence in the asher is provided in Equation (1). 

K K K

K

A E

M F



  (1) 

where 

F plasma asher effective AO fluence (atoms/cm2) 

MK mass loss of Kapton H witness sample (g) 

AK exposed surface area of Kapton H witness sample (cm2

) 

K density of Kapton H (1.4273 g/cm3

) (Ref. 5) 

EK erosion yield of Kapton H (3.00×10–24 cm3

/atom) (Ref. 8) 

As mentioned, two tests were conducted, each with samples from a different Weld Side: Test 1 included 

Side B samples and Test 2 included Side A samples. For each test, the samples were exposed to six 

different individual AO exposures and their Ey were determined. Exposures 1 and 2 were 3.5 hr each, 

exposure 3 was 7 hr, exposure 4 was 14 hr, and exposures 5 and 6 were 28 hr each, for a total exposure of 

84 hr per test. The fluence values of Test 1 ranged from 1.5×1019 atoms/cm2 for Exposure 1 to a 

cumulative fluence of 1.3×1021 atoms/cm2 (depending on position). The fluence values of Test 2 ranged 

from 1.9×1019 atoms/cm2 for Exposure 1 to a cumulative fluence of 1.4×1021 atoms/cm2

. 

Figure 3.—Sample holder with 23 Kapton H 

samples and position identifications. 
NASA/TM—2012-217646 6 

Flux Tests 

The AO fluence of the individual test samples was calculated because flux can vary with position in 

the asher. A Kapton H flux test was conducted to determine the effective flux of each of the 23 positions, 

with a Kapton H witness sample in each position of the sample holder. Since the fluence of a Kapton H 

sample can be determined using Equation (1), and because flux is simply fluence divided by time in 

seconds, the effective flux for each position was determined based on mass loss of each individual 

Kapton H sample. Twenty-two k constants, one corresponding to each of the 22 positions in the holder, 

were then calculated, compensating for the position-based flux variation relative to the central 2.54 cm 

diameter Kapton H witness sample (k = 1). The k constants were then used to calculate the precise flux 

and fluence each sample was exposed to in every test. The k constants were based on the positional flux 

relative to the Kapton H sample flux in position Y, as shown in Equation (2): 

n n k f  k  f (2) 

where 

 fn flux of the sample in position n (atoms/cm2sec) 

 n sample holder position 

kn constant, a fraction of the fluence of a sample in position n relative to the fluence of the 

Kapton H in position Y (kn = 1 for Kapton H) 

fk flux of Kapton H witness sample

Two separate flux tests were conducted and the resulting k values for each sample position were 

averaged. In order to calculate the fluence each sample was exposed to during each individual test, the 

effective fluence was determined for the Kapton H witness sample in position Y and then multiplied by 

the average k factor for each position to obtain the fluence for each sample position. 

Exposure Area Measurements 

The exposure area of each sample opening in the metal holder was calculated by taking diameter 

measurements in three orientations using Fowler & NSK Max-Cal electronic digital calipers and 

averaging the values. For the irregularly shaped samples, the exposed surface area was found by tracing 

the exposed perimeter of the samples (from photographs of the samples taken with a scale bar) using 

AutoCAD, a computer aided design software program. An example of this technique can be seen in 

Figure 4, where the perimeter of the surface area is outlined in red. 

Figure 4.—Photograph of HST-BSTS 

30 sample (position N) prior to the 

first exposure showing the AutoCAD 

trace line used for surface area 

calculation. 
NASA/TM—2012-217646 7 

Erosion Yield Calculation 

Once mass loss, fluence, density, and exposure area values were obtained, the erosion yield (Ey) was 

calculated using Equation (3): 

  A F

M E

S S

S

y



  (3) 

where 

∆MS mass loss of sample (g) 

 AS exposed surface area of sample exposed to atomic oxygen (cm2

) 

S density of sample (g/cm3

)

 F plasma asher Kapton H effective AO fluence (atoms/cm2

)

Photo-Documentation 

Close-up photographs were taken of the irregularly shaped samples before and after each exposure 

using a Canon digital camera mounted on a Landcamera stand so that surface area measurements could be 

obtained using AutoCAD. 

Results and Discussion 

The solar-facing side of the BSTS was found to be extremely embrittled with numerous throughthickness cracks and pieces of insulation material missing in some places. Visually, the anti-solar side of 

Weld 15 did not appear to be degraded. These observations are consistent with mechanical property 

degradation studies conducted by de Groh et al. (Ref. 5) on other Welds from the 20-Weld section 

provided by ESA. 

Material Degradation 

Figures 5(a) and (b) provide photographs of solar facing HST BSTS sample 299° (position M) from 

Test 2 wrapped in its Al holder in the sample holder prior to AO exposure and after the fifth AO 

exposure, respectively. This sample provides an example of a sample that was excessively degraded after 

the fifth exposure. The pre-exposure surface area was 0.153 cm2

, while the surface area after Exposure 5 

was 0.141 cm2

. The blue ovals in the images highlight a distinguishing feature in the small Al holder, 

while the excess erosion can be seen in Figure 5(b) in the upper left section of the sample where a piece 

of material is missing. 

 (a) (b) 

Figure 5.—HST BSTS sample 299 (position M) from Test 2 in small Al 

holder in sample holder: a) prior to AO exposure, and b) after the fifth 

AO exposure. 
NASA/TM—2012-217646 8 

Although both the pristine samples (pristine Al-FEP and pristine BSTS) and the anti-solar-facing 

samples became hazy-white with AO exposure, none of these samples were found to be excessively 

degraded after the full AO exposure. Some of the solar-facing samples that were excessively eroded, such 

as HST BSTS sample 10°, appear to have warped and/or curled during exposure. This warping would 

lead to the post-exposure photographs misrepresenting their surface area, resulting in their true surface 

areas differing from the ones computed. As this would introduce error into the Ey values, it was 

concluded that the cumulative Ey values should not include these higher fluence exposures. 

Fluence and Erosion Yield Values 

Seven of the samples in Test 1 were found to be excessively degraded after the sixth exposure and 

three of the samples in Test 2 were found to be excessively degraded after the fifth exposure. Therefore 

the cumulative Ey for the Test 1 samples is based on data from Exposures 1-5, and the cumulative Ey for 

the Test 2 samples is based on data from Exposures 1-4. It should be noted that in a couple of the 

individual exposures, there were a few surprisingly low mass loss values. In these situations (BSTS 

sample 65° in Test 1, BSTS sample 299° in Test 2, and Pristine Al-FEP (Position C in Test 1)) the 

fluences for those individual test exposures were not included when computing the cumulative Ey values. 

The Polar Plot shown in Figure 6 shows Ey versus solar angle for the HST BSTS samples, along with 

average Ey values for the pristine Al-FEP samples and pristine HST BSTS samples (represented as 

dashed and solid lines, respectively). 

As can be seen in Figure 6, the asher Ey values of the anti-solar samples are slightly lower (average 

Ey of 1.48×10–24 cm3

/atom) than the average Ey values for the pristine BSTS and pristine Al-FEP, which 

were found to be 1.66×10–24 cm3

/atom and 1.63×10–24 cm3/atom, respectively. Although there is a fair 

amount of scatter in the Ey of the solar-facing samples, the majority of solar-facing Ey values are higher 

0

30

60

90

120

150

180

210

240

270

300

330

0.0

5.0e-25

1.0e-24

1.5e-24

2.0e-24

Side B Exp 1-5

Average Pristine BSTS 

Average Pristine Al-FEP 

Side A Exp 1-4 

Figure 6.—Polar Plot of Ey versus solar angle for pristine and HST BSTS samples. 

0 indicates 

the solarfacing side
NASA/TM—2012-217646 9 

than those for the anti-solar or pristine samples. The average Ey for side B of the solar facing HST BSTS 

sample was calculated as 1.79×10–24 cm3

/atom, and side A was 1.94×10–24 cm3/atom. The average Ey for 

the solar facing HST BSTS sample was computed to be 1.86×10–24 cm3

/atom, and the Anti-solar side was 

calculated as 1.48×10–24 cm3

/atom (Test 1 exposures 1-5 and Test 2 exposures 1-4). There was a 

26 percent difference between HST BSTS Average Solar and Average Anti-Solar Ey. 

Another observation that can be made from Figure 6 is that the Ey of samples centered around the 

270° solar-grazing angle (samples 270°, 290°, 285°, and 240°) are lower than all the other Ey values. This 

is particularly true for the HST BSTS 270° sample, which had an Ey of only 7.10×10–25 cm3

/atom, only 

half the Ey of the HST BSTS solar-grazing sample 90° of 1.48×10–24 cm3

/atom. After examining the HST 

BSTS 270° sample under the microscope, it was observed that this sample has a patchy appearance and 

contains some crazed areas, as shown in Figure 7. Therefore, it appears that this sample may contain a 

layer of contamination. The orientation of the sample was such that silicone contamination from the 

silicone on the anti-solar side of the array could be possible. 

In a study by de Groh et al., pristine, solar-facing, anti-solar-facing and solar-grazing BSTS samples 

were examined with scanning electron microscopy for surface morphology and with energy dispersive 

spectroscopy (EDS) for surface chemistry. The pristine, solar-facing and anti-solar-facing surfaces did not 

have any evidence of contamination, with only C and F peaks observed (Ref. 5). However, a crazed 

surface texture was observed on one of the solar-grazing samples (Ref. 5). The EDS data indicate the 

crazed texture is due to silicone contamination (the Pd peak is from the conductive coating) (Ref. 5). The 

back surfaces of the solar arrays were coated with DC 93-500 silicone, so it appears that the solar-grazing 

surface facing the array received silicone contamination from the solar arrays, which impacted the asher 

Ey values in this test. If the anti-solar samples also received very small amounts of contamination on-orbit 

(perhaps below the detection limit of EDS), this would explain the slightly lower Ey values of the antisolar samples compared with those of the pristine samples. 

It should be noted that the erosion yield of Teflon FEP in a plasma asher can be substantially higher 

that the erosion yield in LEO. For example, the erosion yield of FEP determined after 4 years of space 

exposure on the International Space Station as part of the Materials International Space Station 

Experiment 2 (MISSE-2) was found to be 2.00×10–25cm3

/atom, (Ref. 11) which is an order of magnitude 

lower than the Ey determined in this study using a plasma asher. It should also be noted that de Groh et al. 

have shown a correlation of increased erosion yield with increased ESH for Teflon FEP from numerous 

spaceflight experiments (Ref. 12). 

In summary, the average Ey values of the solar-facing HST BSTS samples were found to be 

26 percent higher than the Ey of the anti-solar-facing samples, and 12 to 15 percent higher than pristine 

BSTS or Al-FEP samples, respectively. These results indicate that solar exposure affects the post-flight 

erosion rate of FEP in a plasma asher. Therefore, it likely affects the erosion rate while in LEO. It is 

important that spacecraft engineers consider the enhanced erosion of Teflon FEP on solar-facing surfaces 

when designing durable spacecraft components. 

(a) (b) 

Figure 7.—Solar-grazing Sample 270: (a) both the exposed and protected (upper right) areas 

of the sample can be seen, and (b) close-up showing the crazed surface. 

 
NASA/TM—2012-217646 10 

Summary 

To determine whether solar exposure affects the rate of atomic oxygen erosion of Al-FEP, a very 

commonly used spacecraft thermal control material, ground-laboratory AO experiments have been 

performed on space exposed FEP. The material studied was a section of Al-FEP thermal shield exposed 

to space on the HST for 8.25 years. In these investigations, samples were sectioned from the circular 

thermal shield and exposed to AO in an air-operated radio-frequency plasma asher for Ey determination 

versus on-orbit solar angle. The Ey values of 24 HST BSTS samples were determined in a series of two 

separate tests, with the Ey values compared relative to Kapton H in an asher. Along with pristine BSTS 

and Al-FEP samples, the Ey values of the HST samples were determined for six different exposure tests. 

Cumulative Ey were calculated, though, because error had to be taken into account; the most accurate data 

excluded the last exposure for Test 1 and the last two exposures for Test 2. The average Ey of the solar 

facing HST Al-FEP was found to be 1.9×10–24 cm3

/atom, while the average Ey of the anti-solar HST 

Al-FEP was 1.5×10–24 cm3

/atom. The Ey of the pristine samples was 1.6–1.7×10–24 cm3/atom. These 

results indicate that radiation and solar exposure affects the post-flight erosion rate of FEP in a plasma 

asher. Therefore, it likely affects the erosion rate while in LEO. 

In addition, it was determined that one of the solar-grazing surfaces (270°) was contaminated on-orbit 

with silicone contamination, likely the surface facing the solar arrays, which impacted the erosion yield of 

samples in that orientation. Last, the erosion yield versus fluence comparisons shows that the rate of 

erosion of all samples is greater at the surface of the sample, and that solar-facing samples in particular 

have higher rates of erosion than other samples for fluences below 5.0×1020 atoms/cm2

. By using this 

fluence and the solar facing erosion yield determined in this study, it appears that solar radiation reacted 

with the FEP to a depth of 9 m. 

References 

1. Dickerson, R.E., Gray, H.B., and Haight, G.P., Chemical Principles 3rd Edition, Benjamin 

Cummings Publishing Co. Inc. Menlo Park, CA, 1979, p. 457. 

2. Banks, B.A., Miller, S.K., and de Groh, K.K., AIAA-2004-5638; also NASA/TM-2004-213223, 

2004. 

3. Dever, J.A., NASA Technical Memorandum-103711. 1991. 

4. J.A. Dever, B.A. Banks, K.K. de Groh and S.K. Miller, Degradation of Spacecraft Materials, in 

Handbook of Environmental Degradation of Materials, Myer Kutz (editor), William Andrew 

Publishing, pp. 465–501, 2005. 

5. de Groh, K.K., Finlay, K.A., Snyder, A., High Performance Polymers 20 (2008) 410-428. 

6. (pers. comm. M. Van Eesbeek, ESA, 2004) 

7. Dever, J.A. and de Groh, K.K., NASA/TM-2002-211364, 2002. 

8. Banks, B.A., “The Use of Fluoropolymers in Space Applications,” Ch. 4 in Modern 

Fluoropolymers: High Performance Polymers for Diverse Applications, John Wiley & Sons, Ltd., 

1997. 

9. Hedin A.E. 1987, MSIS-86 thermospheric model J. Geophys. Res. 92 4649-62. 

10. Rockwell International Corporation. (1990). Rocketdyne Division, Seal Beach, CA, USA, 

Specification RC 1800 Review C, p.18, 1990. 

11. de Groh, K.K., Banks, B.A., McCarthy, C.E., Rucker, R.N., Roberts L.M. and Berger, L.A., High 

Performance Polymers 20 (2008) 388-409. 

12. de Groh, K.K., Banks, B.A., Guo, A., Ashmead, C.C., Mitchell G.G. and Yi., G.T., “MISSE 6 

Polymers Atomic Oxygen Erosion Data,” Presented at the 2010 NSMMS held June 28-July 1, 2010 

in Scottsdale, AZ. 

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Effect of Solar Exposure on the Atomic Oxygen Erosion of Hubble Space Telescope 

Aluminized-Teflon Thermal Shields 

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Guo, Aobo; Ashmead, Claire, C.; de Groh, Kim, K.; Sechkar, Edward, A. 

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WBS 315404.07.20.12 

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14. ABSTRACT

When exposed to low Earth orbital (LEO) environment, external spacecraft materials degrade due to radiation, thermal cycling, 

micrometeoroid and debris impacts, and atomic oxygen (AO) interaction. Collisions between AO and spacecraft can result in oxidation of 

external spacecraft surface materials, which can lead to erosion and severe structural and/or optical property deterioration. It is therefore 

essential to understand the AO erosion yield (Ey), the volume loss per incident oxygen atom (cm3/atom), of polymers to assure durability of 

spacecraft materials. The objective of this study was to determine whether solar radiation exposure can increase the rate of AO erosion of 

polymers in LEO. The material studied was a section of aluminized-Teflon (DuPont) fluorinated ethylene propylene (Al-FEP) thermal 

shield exposed to space on the Hubble Space Telescope (HST) for 8.25 years. Retrieved samples were sectioned from the circular thermal 

shield and exposed to ground laboratory thermal energy AO. The results indicate that the average Ey of the solar facing HST Al-FEP was 

1.9×10-24 cm3/atom, while the average Ey of the anti-solar HST Al-FEP was 1.5×10-24 cm3/atom. The Ey of the pristine samples was 1.6-

1.7×10-24 cm3/atom. These results indicate that solar exposure affects the post-flight erosion rate of FEP in a plasma asher. Therefore, it 

likely affects the erosion rate while in LEO. 

15. SUBJECT TERMS

Hubble Space Telescope (HST); Thermal control insulation; Space environment; Environmental degradation; Polymer; Atomic 

oxygen (AO); Solar radiation 

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